1. Field of the Invention
The invention relates in general to a Viterbi decoder in an optical disc system. More particularly, the invention relates to a Viterbi decoder having two different decoding modes in an optical disc system.
2. Description of the Related Art
In a partial response maximum likelihood (PRML) system of an optical disc system, such as a digital video disc (DVD) player, a transmission channel with a memory can be described by a trellis diagram. As shown in FIG. 1A, a block diagram of a conventional fixed level Viterbi decoder with a digital partial response maximum likelihood equalizer is illustrated. The data of a compact disc is read by an optical reading system (not shown) and sent to the analog/digital converter 110. The read data is converted from analog to digital, and sent to the fixed level Viterbi decoder 114 via the digital partial response maximum likelihood equalizer 112. In another method as shown in FIG. 1B, the data of compact disc read by an optical reading system is sent to the analog/digital converter 122 via the analog partial response maximum likelihood equalizer 120. Being converted from analog to digital, the data is then sent to the fixed level Viterbi decoder 124.
In both FIGS. 1A and 1B, a fixed signal level mode is used in the conventional Viterbi decoder. The drawback of this mode is that the partial response maximum likelihood adaptive equalizer is required, and it is difficult to design such partial response maximum likelihood equalizer. To have signal level of the Viterbi decoder suitable for decision feedback method, the convergence time is concerned. Especially, the capability and speed to mend the defects of the received signal are the major problems.
FIG. 1C shows a block diagram of a conventional Viterbi decoder with an adaptive signal level. In FIG. 1C, the data of a compact disc read by an optical reading system is sent to the analog/digital converter 130. The analog/digital converter 130 converts the analog data into digital analog which is sent to the adaptive level Viterbi decoder 132, the estimation unit 134 and the received signal delay chain 136 simultaneously. The estimation unit 134 is used as a decision mechanism. The signal level adaptive algorithm 138 receives the decision bit output by the adaptive level Viterbi decoder 132, the decision bit output by the estimation unit 134, and the delay signal output by the received signal delay chain 136. A signal level is then output to the adaptive level Viterbi decoder 132.
As mentioned above, the signal of the Viterbi decoder is adaptable. However, the adaption has to be delayed until the Viterbi decoder decodes the received signal. Thus, the training cycle is long, and a large amount of registers have to be used to store the delay signals. When serial errors occurs to the Viterbi decoder, a very long time is consumed to mend the internal operation sequence of the Viterbi decoder.